This invention relates to apparatus for imaging sections of a body by transmitting ultrasonic energy into the body and determining the characteristics of the ultrasonic energy reflected therefrom and, more particularly, to an improved ultrasonic scanning head for such apparatus.
In recent years ultrasonic techniques have become more prevalent in clinical diagnosis. Such techniques have been utilized for some time in the field of obstetrics, neurology and cardiology, and are becoming increasingly important in the visualization of a number of different body portions, for example, the scanning of breasts to detect tumors.
Various fundamental factors have given rise to the increased use of ultrasonic techniques. Ultrasound differs from other forms of radiation in its interaction with living systems in that is has the nature of a mechanical wave. Accordingly, information is available from its use which is of a different nature than that obtained by other methods and it is found to be complementary to other diagnostic methods, such as those employing X-rays. Also, the risk of tissue damage using ultrasound appears to be much less than the apparent risk associated with ionizing radiations such as X-rays.
The majority of diagnostic techniques using ultrasound are based on the pulse-echo method wherein pulses of ultrasonic energy are periodically generated by a suitable piezoelectric transducer such as a lead zirconate-titanate ceramic. Each short pulse of ultrasonic energy is focused to a narrow beam which is transmitted into the patient's body wherein it eventually encounters interfaces between various different structures of the body. When there is a characteristic impedence mismatch at an interface, a portion of the ultrasonic energy is reflected at the boundary back toward the transducer. After generation of the pulse, the transducer operates in a "listening" mode wherein it converts received reflected energy or "echoes" from the body back into electrical signals. The time of arrival of these echoes depends on the ranges of the interfaces encountered and the propagation velocity of the ultrasound. Also, the amplitude of the echo is indicative of the reflection properties of the interface and, accordingly, of the nature of the characteristic structures forming the interface.
There are various ways in which the information in the received echoes can be usefully presented. In one common technique, the electrical signal representative of detected echoes are amplified and applied to the vertical deflection plates of a cathode ray display. The output of a time-base generator is applied to the horizontal deflection plates. Continuous repetition of the pulse/echo process in synchronism with the time-base signals produces a continuous display, called an "A-scan, " in which time is proportional to range, and deflections in the vertical direction represent the presence of interfaces. The height of these vertical deflections is representative of echo strength.
Another common form of display is the so-called "B-scan" wherein the echo information is of a form more similar to conventional television display; i.e., the received echo signals are utilized to modulate the brightness of the display at each point scanned. This type of display is found especially useful when the ultrasonic energy is scanned transverse the body so that individual "ranging" information yields individual scan lines on the display, and successive transverse positions are utilized to obtain successive scan lines on the display. The two-dimensional B-scan technique yields a cross-sectional picture in the plane of the scan, and the resultant display can be viewed directly or recorded photographically or on magnetic tape.
While successes have been achieved in the field of ultrasonic imaging, there are a number of problems which need to be overcome in obtaining high quality untrasonic images in a convenient, reliable and cost-effective manner. Regarding problems which have been partially overcome, it is known, for example, that ultrasound is almost totally reflected at interfaces with gas. This has led to the use of coupling through a fluid such as water or the use of a direct-contact type of transducer. The latter technique may give rise to problems when attempting to image structures such as arteries which may be only a few millimeters below the skin surface, the contact imaging causing aberrations in the near field of the transducer. Coupling through a fluid offers advantage over direct-contact in this respect, but, until recently, lead to various design problems and cumbersome generally nonportable structures which were inconvenient to use, especially when attempting to register them accurately on a patient. Some techniques involve immersing the patient in water or obtaining appropriate contact of the body part with a bulky water tank wall.
The need to scan the ultrasonic beam in two dimensions gives rise to problems of bulkiness and difficulty of handling in the scanning unit. In the U.S. Pat. No. 4,084,582, there is disclosed a type of apparatus which provides improved convenience as compared to most water coupled imaging techniques. The apparatus disclosed therein has a console which typically includes a timing signal generator, energizing and receiving circuitry, and a display/recorder for displaying and/or recording image-representative electronic signals. A portable scanning head or module, suitable for being hand held, has a fluid-tight enclosure having a scanning window formed of a flexible material. A transducer in the portable scanning module converts energy from the energizing circuitry to ultrasonic energy and also converts received ultrasound echoes back into electrical signals which are coupled to the receiver circuitry. A focusing lens is coupled to the transducer, and a fluid, such as water, fills the portable scanning module in the region between the focusing lens and the scanning window. A reflective scanning mirror is disposed in the fluid, and a driving motor, energized in synchronism with the timing signals, drives the scanning mirror in periodic fashion. The ultrasound beam is reflected off the scanning mirror and into the body being examined via the scanning window. Improved versions of the scanning head are set forth in my copending U.S. application Ser. Nos. 890,377 and 890,378, assigned to the same assignee as the present application. The scanning head disclosed in these copending applications has, among other features, a scanning window formed of a rigid material.
For a two dimensional B-scan taken with the referenced type of scanning head, the dimensions scanned are: (1) depth into the body, which varies during each display scanline by virtue of the ultrasound beam travelling deeper into the body as time passes; and (2) a slower transverse scan caused by the scanning mirror. The display is typically in a rectangular format, e.g., the familiar television type of display with linear sweeps in both directions. However, the transverse scan of the ultrasound beam itself, as implemented by the scanning mirror, results in a sector scan. For distances deeper in the body, the fanning out of the sectors results in geometrical distortion when displayed on a linear rectangular display. For example, the azimuth dimension in the extreme far field may be, say 21/2 times larger than the azimuth dimension in the extreme near field. Thus, the density of information on the left-hand side of a conventional left-to-right display is much higher than the density of information on the right-hand side of the display. In other words, what appear to be equal distances in the body on the left and right hand sides of the display are actually substantially different distances.
It is among the objects of the present invention to provide a solution to the problem of geometrical distortion, as set forth below.